The invention pertains to a flat product of metal, in particular, in the form of a wall for a heat exchanger which consists of a composite material comprising a titanium or titanium alloy layer and a copper alloy layer.
The invention also pertains to a heat exchanger, the wall of which can be exposed to a cooling medium in the form of a liquid that contains biological organisms, wherein said wall consists of a copper alloy layer that faces the cooling medium and a titanium or titanium alloy layer that tightly adjoins the copper alloy layer and faces away from the cooling medium.
Heat exchangers of this type are known in the form of tubular elements (GB 2 151 744 A). In this known heat exchanger, the tube walls consist of two layers, wherein the inner tube consists of copper or a copper alloy and the outer tube consists of titanium. Both tubes have the same wall thickness of 0.5 mm. The inserted inner tube is hydraulically expanded in order to bring the inner tube in contact with the outer tube over its entire surface.
The development of such a tubular heat exchanger with double layer walls was based on the notion that, although titanium provides an excellent protection against corrosion, biological organisms are deposited or settle down (biofouling) on its contact surface with cooling mediums in the form of liquids that contain biological organisms, e.g., seawater or brackish water. This is caused by the distinctive biocompatibility of titanium and impairs the heat transmission. In extreme instances, it may even occur that the heat exchanger becomes clogged. These disadvantages can be eliminated if the inside of the titanium tube is lined with a copper tube, namely because copper has a toxic effect on biological organisms. However, it is disadvantageous that the copper or copper alloy only provides a weak protection against corrosion when it comes in contact with aggressive mediums such as seawater (0.5 mm erosion/year), and that the heat transmission from the inner copper tube to the outer titanium tube is also more or less impaired on the boundary between the copper tube and the titanium tube if both tubes tightly adjoin one another (high heat transmission resistance).
In another heat exchanger of this type which is realized in the form of a multi-tube heat exchanger, the tubes of titanium are connected to a tube receptacle plate of a copper alloy (JP 60 11 70 99 A). In case of an interruption in the operation of the heat exchanger, the water circulation is shut down such that the protection against electric corrosion between the different metals of the tubes (titanium) and the tube receptacle plate (copper alloy) is no longer effective. In order to prevent an undesirable electric corrosion in such instances, sacrificial electrodes that are in intimate and electrically conductive contact with the titanium tubes and consist of the same copper alloy as the tube receptacle plate are arranged on the surface of the titanium tubes. If the water circulation is shut down, the occurring electric corrosion only takes place between the titanium tubes and the sacrificial electrodes such that the undesirable electric corrosion between the tubes and the tube receptacle plate is prevented. The sacrificial electrodes are replaced as required such that a permanent protection against electric corrosion between the titanium tubes and the tube receptacle plate is ensured.